Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used for various types of optical materials. These optical materials are required to have low specific gravity, high transparency and low yellowness, and as optical properties, high refractive index, high Abbe number, etc. In order to satisfy these requirements, many compositions for optical materials comprising a sulfur-containing monomer, in particular, a polythiol compound have been developed (Patent Documents 1-2). However, in the case of the polythiol compounds described in the documents, an insoluble component (suspended matter) may be generated to cause turbidity during preservation, and white turbidity may be generated in resins obtained by polymerization and curing using these polythiol compounds.